1. Field of the Invention
The present invention relates to a curtain coating method and a curtain coating apparatus, specifically a curtain coating method and a curtain coating apparatus in which at least one layer of a coating liquid is ejected from a slit, and the ejected coating liquid is made to fall freely by using a curtain edge guide, which guides the coating liquid in the form of a curtain liquid film, so as to apply the coating liquid onto a continuously running web.
2. Description of the Related Art
Curtain coating methods are coating methods frequently used in producing photosensitive materials and the like, for example photographic films. Among the curtain coating methods, for example, there is a method which includes ejecting a coating liquid from a nozzle slit of a curtain coating head 1, making the ejected coating liquid fall freely by using a curtain edge guide 2, which guides the coating liquid in the form of a curtain liquid film, so as to form a curtain liquid film 3, and bringing the curtain liquid film 3 into contact with a continuously running web 5 so as to form a coating film on the web, as shown in FIG. 1, and there is a method which includes ejecting a coating liquid from a slit, moving the ejected coating liquid on a slide surface 7, making the coating liquid fall freely by using a curtain edge guide 2, which guides the coating liquid in the form of a curtain liquid film, so as to form a curtain liquid film 3, and bringing the curtain liquid film 3 into contact with a continuously running web 5 so as to form a coating film on the web, as shown in FIG. 2. Also, as for multilayer coating, there is a method which includes ejecting coating liquids with various functions from respective nozzle slits, making the ejected coating liquids fall freely by using a curtain edge guide which guides the coating liquids in the form of a curtain liquid film, and bringing the curtain liquid film into contact with a continuously running web so as to form a coating film on the web, and there is a method which includes ejecting coating liquids with various functions from respective slits, depositing the ejected coating liquids on a slide surface, making the deposited coating liquids fall freely by using a curtain edge guide which guides the coating liquids in the form of a curtain liquid film, and bringing the curtain liquid film into contact with a continuously running web so as to form a coating film on the web.
Parenthetically, in such coating film forming methods, there is a phenomenon caused in which when a coating liquid flows along a curtain edge guide, the coating liquid flows slowly at both edges of the formed curtain liquid film supported by the curtain edge guide, and the coating liquid at the edges of the curtain liquid film flows in a manner that is shifted toward the central side owing to the difference in flow speed between the coating liquid at the edges of the curtain liquid film and the coating liquid on the central side of the curtain liquid film. Thus, when the coating liquid is made to fall freely and the formed curtain liquid film is brought into contact with a continuously running web so as to form a coating film on the web, there is such a drawback that the amount of the coating liquid attached becomes larger at the edges of the coating film with respect to the width direction. Consequently, there are undried portions easily existing when the coating film is dried, which causes blocking when a product is wound, and the edges swell, which causes cutting of the web when the product is wound, thereby lowering production efficiency.
As an attempt to prevent the foregoing, the drying temperature may be increased. However, there is such a problem that, regarding coating on thermosensitive paper, the thermosensitive paper develops color when the temperature of a coating film is high, thereby causing defects in products. Thus, increasing the drying temperature is not helpful in many cases.
To prevent the phenomenon in which the amount of the coating liquid attached becomes larger at the edges of the coating film, there is a well-known method which includes making the coating liquid fall freely while pouring an auxiliary liquid along edge portions at both ends of the curtain edge guide which support the formed curtain liquid film, thereby making the flow speed of the coating liquid at the edges closer to the flow speed of the coating liquid at the center (refer to Japanese Patent Application Laid-Open (JP-A) Nos. 2000-513, 2000-218209, 2001-104856, 2005-512768 and 2008-93656, for example). As shown in FIG. 3, the auxiliary liquid (auxiliary liquid 9) is sucked at a bottom of the curtain edge guide 2 and thus recovered.
However, when the auxiliary liquid is recovered, a small amount of the coating liquid is also recovered, thereby causing a residue (S) of the liquid to accumulate on a claw 10 and in a suction port 11, as shown in FIG. 4. This is a phenomenon caused because when the auxiliary liquid 9 is recovered, air flows fast in places owing to suction on the web advancing direction side of the curtain liquid film on a coating liquid contacting surface of the claw 10 and on the opposite side to the web advancing direction side, and thus the slowly flowing coating liquid in contact with the claw dries. The residue (S) accumulates with time on the claw 10 placed at a bottom of the curtain edge guide 2 at the time of continuous production, the curtain liquid film 3 becomes unable to be supported by an edge of the claw 10 owing to the residue (S), causing the curtain liquid film 3 to deviate inward, and thus the amount of the coating liquid attached becomes larger at the edges of the coating film with respect to the width direction. Consequently, the uneven coating width at the time of production leads to great production loss. Also, since the foregoing amount becomes larger, there are undried portions easily existing due to insufficient drying at the time of production, the coating liquid is possibly attached to a conveyance roll of the web during the production, later smearing the coating film surface of the web, blocking possibly arises when a product is wound, and the web is possibly cut because of the swollen edges when the product is wound, thereby lowering production efficiency.
To prevent swinging and inward deviation of a curtain liquid film in a curtain coating method, JP-A No. 11 -188299 (Troller Schweizer Engineering) discloses a curtain coating method which includes using a porous material for a curtain edge guide, and evenly pouring an auxiliary liquid onto a surface provided in contact with a curtain coating liquid in the curtain edge guide. Meanwhile, JP-A No. 2001-46939 (MITSUBISHI PAPER MILLS LIMITED.) discloses a curtain coating method which includes using a plate of glass for a surface provided in contact with a curtain coating liquid in a curtain edge guide. However, neither of these (JP-A No. 11-188299 and JP-A No. 2001-46939) discloses removal of a residue of the coating liquid and the auxiliary liquid at the bottom of the curtain edge guide.
FIG. 18A schematically shows a state in which a curtain liquid film does not deviate inward, and FIG. 18B schematically shows a state in which a curtain liquid film deviates inward.